One click.
One click, if you're willing to wait 60 seconds. Just press the power button on your computer and walk away. With the way people go on about how many clicks they need to do things, I get the impression that would be preferable to them. For what it's worth, I think I clicked the left mouse button 8 times in the process of writing this comment, and I definitely do that more often than shutting down my computer. No kittens have died, and my hand is completely intact.